In general, polybenzimidazole-based polymers have higher density as compared to other aromatic polymers, and exhibit a thermal decomposition temperature of 500° C. or higher and a glass transition temperature of 400° or higher. Thus, such polybenzimidazole-based polymers having the highest thermal, chemical and physical stability among the polymers known to date have been used in various industrial fields.
However, while such polybenzimidazoles are highly crystalline and have excellent mechanical strength, they show poor solubility to organic solvents and low permeability to substantially all types of gases, and thus are limited in application to gas separation membranes. It is known that this is because the hydrogen molecules present on the polymeric chain cause hydrogen bonding between one chain and another, so that the polymer structure becomes rigid and the polymeric chains are packed closely. Therefore, some researchers have conducted many studies to inhibit such packing of polymeric chains and to reduce the rotational movement of polymeric chains. Additional studies have conducted about modification of the polymeric backbone to increase the free volume between one polymeric chain and another, and thus to improve gas permeability.
One of the above studies has made an attempt to increase solubility of polybenzimidazole in order to overcome the problem of poor processability and limited application of polybenzimidazole caused by low solubility characteristics unique to polybenzimidazole. Particularly, P. R. Srinivasan et al. have reported that polybenzimidazole obtained by introducing fluorene as a cardo group to the backbone of polybenzimidazole realizes improved solubility, while maintaining thermal stability thereof (Non-patent Document 1). However, there is no disclosure about forming the polybenzimidazole into a film shape and using the same as a gas separation membrane. It is thought that even though polybenzimidazole may be formed into a film shape, the resultant film is too brittle to apply it to industrial applications due to the structural characteristics of polybenzimidazole.
In addition, it is reported that a composite membrane obtained by coating polybenzimidazole onto the outer surface of a steel support layer (intermediate layer: zirconia) is used to determine the gas permeability (Non-Patent Document 2). However, this is merely about permeability-selectivity of hydrogen and carbon dioxide from hot syngas, and there is no suggestion about oxygen permeability using a polybenzimidazole-based monolayer membrane.
Meanwhile, Guey-Sheng Liou et al. have reported that wholly aromatic polyamide (aramid), polyester (polyarylate) and poly(1,3,4-oxadiazole) having a polymeric backbone to which fluorene is introduced as a cardo group is prepared to improve the solubility of polymer, and the resultant polymer formed into a film shape provides excellent light-emitting properties and may be used as a blue light-emitting material of a polymeric light-emitting device (Non-Patent Document 3). However, there is no suggestion or disclosure about the use of polymer as a gas separation membrane.
Further, a polyimide-based single membrane including cardo groups introduced to a polyimide backbone having thermal properties and mechanical properties similar to those of polybenzimidazole has been determined for its gas permeability. However, this is merely about permeability-selectivity of nitrogen and carbon dioxide (Patent Document 1).
Therefore, the present disclosure provides a gas separation membrane obtained by forming a membrane from polybenzimidazole having significantly improved oxygen permeability by virtue of an increased distance between one polybenzimidazole chain and another, while maintaining high anticorrosive property and thermal/chemical stability. Particularly, it is required that polymers for use in On Board Inert Gas Generation System (OBIGGS) have heat resistance at 90° C. or higher, which is the air temperature of an aircraft engine, and provide excellent oxygen permeability and selectivity over nitrogen, which is inert gas in the fuel tank of an aircraft. Thus, the polybenzimidazole gas separation membrane disclosed herein may be used as a material suitable for OBIGGS.